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Cladocora caespitosa seawater pH pCO 2 Mediterranean ocean acidificationBoron isotopic and elemental systematics are used to define the vital effects for the temperate shallow water Mediterranean coral Cladocora caespitosa. The corals are from a range of seawater pH conditions (pH T~7 .6 tõ 8.1) and environmental settings: (1) naturally living colonies harvested from normal pH waters offshore Levanto, (2) colonies transplanted nearby a subsea volcanic vent system, and (3) corals cultured in aquaria exposed to high (700 μatm) and near present day (400 μatm) pCO 2 levels. B/Ca compositions measured using laser ablation inductively coupled mass spectrometry (LA-ICPMS) show that boron uptake by C. caespitosa cultured at different pCO 2 levels is independent of ambient seawater pH being mainly controlled by temperature-dependent calcification. In contrast, the boron isotope compositions (δ 11 B carb ) of the full suite of corals determined by positive thermal ionisation mass spectrometry (PTIMS) shows a clear trend of decreasing δ 11 B carb (from 26.7 to 22.2‰) with decreasing seawater pH, reflecting the strong pH dependence of the boron isotope system. The δ 11 B carb compositions together with measurements of ambient seawater parameters enable calibration of the boron pH proxy for C. caespitosa, by using a new approach that defines the relationship between ambient seawater pH (pH sw ) and the internally controlled pH at the site of calcification (pH biol ). C. caespitosa exhibits a linear relationship between pH sw and the shift in pH due to physiological processes (ΔpH = pH biol − pH sw ) giving the regression ΔpH Clad = 4.80 − 0.52 × pH sw for this species. We further apply this method ("ΔpH-pH sw ") to calibrate tropical species of Porites, Acropora, and Stylophora reported in the literature. The temperate and tropical species calibrations are all linearly correlated (r 2 N 0.9) and the biological fractionation component (ΔpH) between species varies within~0.2 pH units. Our "ΔpH-pH sw " approach provides a robust and accurate tool to reconstruct palaeoseawater pH sw for both temperate and tropical corals, further validating the boron fractionation factor (α B3-B4 = 1.0272) determined experimentally by Klochko et al. (2006) and the boron isotope pH proxy, both of which have been the foci of considerable debate.

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